Methods of forming and purifying nickel-titanium containing alloys



United States Patent U.S. Cl. 75-135 8 Claims ABSTRACT OF THE DISCLOSUREMethods for forming and treating titanium-nickel alloys which includepreparing the alloys by induction melting the nickel, adding thetitanium to the molten nickel, contacting the resulting alloy withcarbon while under vacuum. The alloys are further cast in suitablegraphite molds with the solidification process proceeding from thebottom toward the top of the mold.

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

This invention pertains to the formation and treatment of alloys andmore particularly to the formation and treatment of alloys comprising ahighly reactive metal and a relatively unreactive metal.

Many techniques and methods have been tried in an effort to find aneffective means for forming alloys containing a highly reactive metal.For example, attempts to form alloys of the highly reactive metaltitanium by fusion techniques were not very successful due to the highreactivity of molten titanium with the interstitial elements, oxygen,hydrogen, nitrogen and carbon. Thus, the use of well known ceramiccrucibles, e.g., SiO A1 0 etc., for forming titanium alloys resulted ina disasterous contamination due to the interaction of the titanium andthe oxygen of the crucible. The use of high purity dense graphitecrucibles was not much more successful since carbide impurities resultedfrom the interaction between the titanium and carbon of the crucible.The use of techniques such as special electrical induction fields tominimize graphite contact and thus minimize carbide formation were alittle more successful, but they are difiicult and costly.

As a result of the failure of fusion methods, the alloys of highlyreactive metals are generally prepared by arcmelting techniques, e.g.,titanium-nickel alloys have been formed by both consumable andnon-consumable methods, employing a water-cooled copper crucible. Thesetechniques, however, sufier from the following disadvantages:

(1) Composition control is difficult due to the lack of stirringcapability in the overall melt.

(2) Little or no opportunity exists for alloy purification duringmelting.

(3) Costly multiple arc-melting operations are necessary to promotechemical homogeneity in the cast ingots.

(4) Only limited cast shapes can be produced.

Accordingly, it is an object of this invention to provide a novel methodfor forming alloys comprising a highly reactive metal and a relativelyunreactive metal.

It is another object to provide a novel method for producing the abovementioned alloys that promotes chemical homogeneity.

It is a further object to provide a novel method for forming and castingthe above mentioned alloys.

It is still another object to provide a novel method for purifying theabove mentioned alloys.

ice

It is still a further object of this invention to provide a novel methodfor forming and casting nickel-titanium alloys.

It is yet another object of this invention to remove oxygen impuritiesfrom nickel-titanium alloys.

These and other objects will become more readily apparent from readingthe following detailed description of the invention.

The method of this invention for forming the alloy generally comprisesmelting the relatively unreactive component of the alloy in a containerformed from a stable material, adding to and melting in the containerthe reactive metal component and pouring the resulting alloy into asuitable mold for solidification. The melting operation is performed inan inert atmosphere, i.e., any atmosphere that excludes atmosphericcontaminants including, for example, a vacuum; a rare gas such as argon,helium, etc., and the like, with the use of a rare gas at atmosphericpressure or higher being preferred since it prevents the leakage of airinto the system.

The container is generally formed from either thoria or magnesia withmagnesia being preferred due to its lower cost and less toxic nature. Itis to be understood, however, that the term container formed from astable material is not limited to containers wholly made from suchmaterials but also includes containers made from other materials thathave their inner surfaces coated with stable materials. The stablematerial used for forming the container generally has a purity of atleast about 97%, with at least about 99% being preferred, in order tolessen the possibilities of oxygen impurities resulting from theinteraction of the reactive metal component with the oxide impuritiesgenerally found in such materials.

The relatively unreactive material is melted in the container firstsince it has been found that the molten unreactive component has amoderating effect upon the reactive metal component which reducesmetal-container interaction. Since this moderating effect isconsiderably reduced when the atomic ratio of reactive component tounreactive component exceeds about 2 to 1, the method of this inventionis most effective on alloys that fall within this ratio.

The components are generally melted in a low frequency induction furnacesince such an operation has a good mixing effect which promotes chemicalhomogeneity. Although low frequency induction melting techniques arepreferred because they produce superior alloys at a lower cost, it is tobe understood that other melting techniques may be employed so long asthey are utilized in an inert atmosphere and the relatively unreactivecomponent is melted first in a container formed from a stable materialsuch as magnesia or thoria.

The method of this invention is generally performed, using anickel-titanium alloy as a representative example, by first placing adry clean magnesia or thoria crucible containing the desired weight ofnickel into a low frequency induction furnace having an inertatmosphere. The nickel is melted and the amount of titanium necessary togive the desired nickel-titanium ratio is charged from a charging chuteinto the molten nickel. The molten titanium and nickel are intimatelymixed in the crucible by the low frequency of the furnace and whenalloying is complete, the molten alloy is charged into a suitable moldfor solidification.

The method of this invention for removing oxygen impurities generallycomprises contacting the molten alloy with carbon under vacuum. Moreparticularly, the alloy may be purified either by induction melting thealloy under a vacuum of at least about 10' mm. in a carbon container,preferably in the form of high density graphite or by induction meltingthe alloy under a vacuum of at least mm. in any suitable containerfollowed by the addition of carbon. Although the invention is not to belimited or bound by any theoretical reactions or equations, it isbelieved that the removal of oxygen impurities is effected, using anickel-titanium alloy as a representative example, by the followingreaction:

The purification effect of the carbon may be enhanced by adding to themolten alloy a metal that both forms an oxide having a high heat offormation and does not alloy with the metals present in the moltenalloy. Thus for example, an excess of calcium or magnesium metal (basedon the amount necessary to combine with the oxygen impurities present)is placed beneath the surface of the molten alloy in the graphitecontainer, said metal and oxygen combining to form an oxide which israked off as a slag. When the purification is completed the excess metalhaving a higher vapor pressure is removed by vacuum techniques.Alternatively, the purification may be promoted by bubbling hydrogeninto the molten alloy in the carbon crucible, said hydrogen combiningwith the oxygen imuprity to form water vapor.

The alloy forming and purification techniques described herein may beperformed on any alloy comprising highly reactive and relativelyunreactive metal components. As representative examples of highlyreactive metal components there may be mentioned Group IV metals such ashafnium, zirconium, titanium, etc., rare earth metals such as cerium,etc., and the like. As representative examples of relatively unreactivemetal components there may be mentioned iron, cobalt, copper, indium,aluminum, nickel, gold, lead, and the like. The methods of thisinvention are particularly applicable to the 50 to 70 weight percentnickel, remainder essentially titanium alloys, especialy thestoichiometric nickel-titanium alloy (53.5 to 56.5 weight percentnickel, the remainder essentially titanium) which are described in moredetail in US. Patent No. 3,174,851, granted Mar. 23, 1965, which ishereby incorporated by reference.

There are a wide variety of embodiments incorporating the teachings ofthis invention which may be utilized in producing a cast shape of analloy comprising a highly reactive and relatively unreactive metal. Onesuch embodiment comprises forming the alloy by the method of thisinvention, pouring the molten alloy before solidification into a carboncrucible, said alloy being kept molten under vacuum to effectpurification and pouring the molten alloy into a suitable mold forsolidification. Alternatively, any suitable crucible may be usedfollowed by the addition of carbon to elfect purification.

Another embodiment comprises placing an ingot prepared by either themethod of this invention or any other method into a graphite crucible,induction melting the alloy under vacuum to effect purification andpouring the molten alloy into a suitable mold for solidification.Alternatively, any suitable crucible may be employed followed by theaddition of carbon to the molten alloy.

A further embodiment comprises pouring the molten alloy prepared by themethod of this invention before solidification through a carbon linedfunnel into a suitable mold for solidification, said operation beingperformed under vacuum.

Still another embodiment for providing intricate cast shapes whileeffecting purification comprises delivering under vacuum the alloyformed by the method of this invention in either liquid or solid form toa specially designed mold made of high purity dense dried graphite. Themold has a suitably matched induction coil around it and the inductioncoil is equipped with taps to regulate heating specific sections of themold. If the charge to the mold is molten, the mold may be preheated toa ternperature in excess of the melting point of the alloy concerned.Once the mold is filled (including the filling of generous, hot top"),the induction coil is gradually closed off starting at the bottom of themold. By solidifying the casting from the bottom of the mold to the topit is possible to produce a minimum shrinkage pipe, and it will be inthe hot top section. Further, by casting in the heated mold it ispossible to provide a fine cast surface, optimize casting definition inthin sections and minimize porosity within the cast section. If thecharge to the mold is solid, the solid alloy is placed in a graphitehopper attached to the top of the mold, said induction coils being woundwell up on the hopper. The mold and hopper is heated by inductioncausing the alloy concerned to melt in the hopper and run into the mold.The solidification is then performed as mentioned above placing theshrinkage pipe in the hopper.

The following examples are illustrative of the invention but they arenot intended to limit it in any manner.

EXAMPLE I A 55 weight percent nickel, remainder essentially titaniumalloy may be prepared in the following manner.

2750 grams of nickel in the form of carbonyl nickel shot (99.9% pure)are placed in a magnesium oxide crucible (99.9% pure). The crucible isplaced in an induction furnace which has an atmosphere of argon at apressure of one atmosphere. The furnace is run at an induction input ofapproximately 3,000 cycles and the nickel in the crucible is heated to atemperature slightly in excess of 1600 C. in order to effect melting.2,250 grams of titanium (in the form of loose sponge) are added to themolten nickel and the temperature is maintained slightly in excess of1500 C. until the titanium and nickel are completely mixed (usually lessthan 5 minutes). The alloy melt is immediately poured into a suitablemold for solidification.

EXAMPLE II A nickel-titanium alloy (55 weight percent nickel, remainderessentially titanium) is purified by placing the alloy in a high densitygraphite crucible which is then placed into an induction furnace. Avacuum of 10* millimeters is pulled and the induction input to thefurnace is approximately 3,000 cycles. The alloy is held at about 1500C. to effect purification and is then poured into a mold forsolidification.

The methods and teachings of this invention are extremely valuable sincethey permit the use of an induction melting technique for forming analloy containing a highly reactive metal, retaining its advantages,i.e., low cost, chemical homogeneity, composition control, etc., whileavoiding the usual disadvantages, i.e., contamination of the resultingalloy, etc. This invention will lbroaden the commercial applicability ofthe aforementioned alloys of US. Patent No. 3,174,851 since theformation and casting of these alloys into intricate, complex shapes isnow economically feasible.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

1. A method for forming and purifying a metal alloy comprising, saidmethod comprising:

(a) induction melting the nickel component in a container formed from amaterial selected from the group consisting of magnesia and thoria, saidinduction melting being performed in an inert atmosphere;

(b) adding to and melting in said container the titanium component;

(0) contacting the molten alloy with carbon, said contacting beingperformed under vacuum; and

(d) solidifying said alloy.

2. The method of claim 1 wherein said vacuum is at least about 10 mm,

3. The method of claim 2 wherein said contacting with carbon isperformed by adding carbon to the molten alloy.

4. The method of claim 2 wherein the container is formed from magnesia.

5. The method of claim 2 wherein said contacting with carbon isperformed by pouring the molten alloy into a graphite container.

6. The method of claim 2 further comprising promoting the purificationby bubbling hydrogen into the molten alloy during the contacting.

7. The method of claim 2 further comprising promoting the purificationby placing a metal selected from the group consisting of calcium andmagnesium beneath the surface of the molten alloy during the contacting.

8. The method of claim 7 wherein the metal is calcium.

References Cited UNITED STATES PATENTS 6 2,564,498 8/1951 Nisbet 75492,580,273 12/1951 Bens et a1 7565 X 2,805,148 9/1957 DeLong 75-65 X1,979,506 11/1934 Umbreit 75135 X 2,776,204 1/1957 Moore 7549 2,815,27312/1957 Moore 7549 2,815,279 12/1957 Moore 7549 X 3,019,102 1/1962Saarivirta 75135 X 3,116,998 1/1964 Pagonis 26634 X 3,137,566 6/1964Thieme 7549 X 3,174,851 3/1965 Buehler et a1. 75-170 3,188,198 6/1965Moore 7549 X OTHER REFERENCES Darmara et 211.: Vacuum Induction Melting,J our. of the Iron & Steel Inst., March 1959, pp. 266, 268 and 272.

HENRY W. TARRING II, Primary Examiner US. Cl. X.R.

vs-ss, s2, 93

